A polyimide resin is a useful engineering plastic that has high thermal stability, high strength and high solvent resistance due to rigidity, resonance stabilization and firm chemical bond of the molecular chain thereof, and is being applied to a wide range of fields. A polyimide resin having crystallinity is further enhanced in the heat resistance, the strength and the chemical resistance thereof, and thus is expected for applications as alternatives of metals or the like.
Meanwhile, a growing demand for highly heat-resistant low colored resins has been growing in recent years. This is because there has been such a growing demand that glass for use in display materials or the like is replaced with a resin, thereby improving weight saving and impact resistance, or a highly white resin is used in a reflector for automobiles, thereby maintaining weight saving and high luminance. Improvement in productivity is also an important factor for these use applications. Thus, a resin having thermoplasticity is also sufficiently highly advantageous.
However, only rare resins possess both high heat resistance and moldability (thermoplasticity) and additionally have low colorability or flame resistance. A large number of studies have therefore been made on, for example, low coloring or imparting of thermoplasticity by use of a polyimide resin, which is originally highly heat-resistant.
For example, Vespel (registered trademark), a highly heat-resistant resin, is known as a polyimide molding material (PTL 1). This resin is difficult to process by molding due to its very low flowability even at a high temperature, and is also disadvantageous in terms of cost because it requires molding under conditions of a high temperature and a high pressure for a prolonged period of time. In contrast to this, a resin having a melting point and flowability at a high temperature, such as a crystalline resin, may be processed by molding easily and inexpensively.
Thus, a polyimide resin having thermoplasticity has been reported in recent years. Such a thermoplastic polyimide resin is excellent in molding processability in addition to the original heat resistance of the polyimide resin. The thermoplastic polyimide resin is therefore applicable to a molded article for use in an inhospitable environment to which nylon or polyester, a general purpose thermoplastic resin, is inapplicable.
A polyimide resin generally has no melting point below the decomposition temperature because of its rigid structure. Aurum (registered trademark) has been put on the market as a crystalline thermoplastic polyimide resin that may be injection-molded or extrusion-molded (see PTL 2 and NPL 1). The material is a rigid wholly aromatic polyimide resin but succeeds to have a melting point, which is generally difficult to be observed, at a temperature lower than the decomposition temperature by introducing plural flexible ether bonds and meta structures into the structure. Also, in light of its glass transition temperature as very high as 250° C., it can be said that the resin is excellent in heat resistance.
Furthermore, a method using a long linear aliphatic diamine as a raw material diamine is one of the methods for improving the molding processability of the polyimide resin, i.e., the methods for decreasing the melting point of the polyimide resin (NPL 2). This reduces the rigidity of the polyimide, and thus also decreases the melting point. This method, however, might decrease the glass transition temperature along with the decrease of the melting point, and in particular, might reduce the strength at a high temperature. Another problem of this method is difficult synthesis of a polyimide resin using a raw material diamine composed mainly of an aliphatic diamine.